FEMTOSECOND TIME-RESOLVED THERMODULATION OF THIN GOLD-FILMS WITH DIFFERENT CRYSTAL-STRUCTURES

A femtosecond laser is used to generate and probe hot electrons in pol ycrystalline and single-crystalline thin gold films. Transient thermor eflectivity and thermotransmissivity for different heating-laser-pulse fluences are performed. Analysis of the transient thermotransmissivit y and thermoreflectivity signals allows us to resolve the modulation t o the real and the imaginary parts of the dielectric constant. The lat ter is found to be dominant at our probe wavelength. The hot-electron energy-loss. lifetime is shown to be 1-3 ps and increases with the las er fluence. For film thickness comparable to the optical skin depth, t he transient decay time of the reflectivity and of the transmissivity are equal, with the polycrystalline films showing a slightly faster de cay time. For thicker films, hot-electron transport to the bulk of the film gives a faster transient-reflectivity decay. Electron transport is slower in the polycrystalline films.